Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Optical properties of blood (e.g., absorptivity at a specified wavelength(s)) can differ from optical properties of surrounding tissue (e.g., skin, the walls of blood vessels); further, optical properties of blood can be related to other properties of the blood (e.g., the oxygen binding state of hemoglobin in the blood). Thus, the presence and/or amount of blood in a tissue, the oxygen saturation of the blood, or other properties of blood and/or tissue can be detected by illuminating the blood and/or tissue and detecting a property of light reflected, refracted, transmitted, scattered, or otherwise emitted by the blood and/or tissue in response to the illumination. A measurement of the volume of blood in a tissue over time (e.g., by measuring the degree to which the tissue and/or blood absorb an illuminating light over time) can be used to determine a pulse rate, a flow profile, a pressure profile, or some other information about perfusion and/or flow of blood in the tissue. A measurement of the amount of illumination that is absorbed by blood at two or more specified wavelengths can allow the determination of an oxygen saturation of the blood.